1. Introduction
This invention relates to an etchant comprising sulfuric acid activated with hydrogen peroxide or a synergistic combination of hydrogen peroxide and hexavalent molybdenum, a process for using the same, especially in the manufacture of printed circuit boards and novel articles made therewith.
2. Description of the Prior Art
The activation of relatively dilute mineral acids with an oxygen release compound is known in the art and described, for example, in Plating, "Surface Treatment of Metals with Peroxygen Compounds", Vol. 42, pg 561 (1955). In the Plating publication, it is taught that mineral acids such as sulfuric acid, nitric acid, acetic acid and phosphoric acid may be activated with oxygen release compounds such as hydrogen peroxide or other peroxy, persulfate or perborate compounds. The oxygen release compound enhances the etch rate thereby permitting a decrease in the concentration of acid.
Activation of dilute sulfuric acid with hydrogen peroxide is most attractive due to low cost and ease of electrolytic copper recovery from the spent etchant prior to disposal. However, the utility of such hydrogen peroxide etching solutions is reduced somewhat due to catalyzed decomposition of hydrogen peroxide caused by etched metal ions or other transition metal ions in solution and a slow etch rate, typically 1.4 mils of copper from 1 ounce copper clad laminate in 10-25 minutes at 120.degree. F. To enhance etch rate, more concentrated solutions of hydrogen peroxide have been used, but concentrated peroxide solutions are hazardous to health and safety. In addition, such etchants have not been successfully used with tin dissimilar metal etch resists because they attack tin, especially immersion tin.
To limit metal ion catalyzed decomposition of hydrogen peroxide, a number of stabilizers are used in the prior art. A variety of such stabilizers is disclosed in U.S. Pat. Nos. 3,293,093; 3,341,384, 3,407,141; and 3,668,131, all incorporated herein by reference. A preferred class of stabilizers is disclosed in U.S. Pat. No. 3,801,512 incorporated herein by reference. The stabilizers disclosed in said patent are the arylsulfonic acids.
Though stabilized acid solutions activated with hydrogen peroxide are improved over their unstabilized counterparts, nonetheless they still possess disadvantages which limit their use, particularly for the manufacture of printed circuit boards.
In commonly assigned copending U.S. patent application Ser. No. 822,002 (filed concurrently herewith), a new etchant is disclosed comprising hydrogen peroxide and a molybdenum compound in an acidic solution. The hydrogen peroxide provides sustained etching with the advantages and disadvantages described above. In the acid/peroxide environment, the molybdenum etches at a substantially greater rate and therefore exalts the rate to a desirable level for commercial use. The molybdenum cannot be used as a sole oxidant because it is rapidly depleted as it is reduced to a lower valent form as etching proceeds. It is therefore incapable of providing sustained etching. Though not wishing to be bound by theory, it is believed that the combination of the peroxide and the molybdenum is a synergistic combination because both are believed to etch though the peroxide in the acid environment is believed to provide the secondary function of oxidizing molybdenum to a higher valence capable of etching metals in the acidified peroxide environment.
The etchant of the above-noted copending application Ser. No. 822,002 is an improvement over the acid etchants activated with peroxide as disclosed in the prior art. However, the etchants of said application are not desirably used in certain specific processes for etching copper, including its alloys, where the copper is etched in the presence of tin as a metal etch resist such as in chemical milling of copper and in the manufacture of printed circuit boards. The limitation in the use of the etchant is a consequence of attack by the etchant on immersion tin and certain electroplated tins resulting in either the complete dissolution of the tin or partial removal of some of the tin and discoloration of the remaining tin.